The delivery of sufficient oxygen and metabolic precursors for the continued function of the heart is critically dependent upon a sufficient blood supply. Unlike most organs of the body the heart extracts almost all available oxygen from the capillary blood and has minimal anaerobic capacity. Any increases in oxygen demand by the heart must therefore be met by an immediate increase in blood flow. Even a brief interruption of blood flow can lead to ischemia. An understanding of the mechanisms by which coronary vascular tone is regulated is thus of immediate importance to our understanding of the physiology and pathology of the heart. The proposed study investigates the role which electromechanical coupling plays in the control of coronary arterial tone. The working hypothesis is that changes in membrane potential play a significant role in modifying the contractile activity of this vessel. The experiments outlined in this grant proposal will investigate the role of electromechanical coupling in the coronary artery during contraction and during relaxation. The experimental design will include the simultaneous measurement of electrical and contractile activity in whole tissue. In addition the cellular basis for these potential changes will be further investigated using whole cell patch clamp techniques in isolated coronary artery cells. Using patch clamp techniques we will control membrane current or potential to identify the characteristics of channels which are involved in electromechanical coupling. Contraction will be initiated with the histamine H1 receptor agonist 2-(2-aminoethyl)pyridine (AEP), a powerful contractile stimulus of the coronary artery known to cause depolarization. The specific topics to be considered in this application include: 1) What is the contraction- voltage relationship for AEP or elevated K+ in whole tissue? 2) What is the relaxation-voltage relationship for several different hyperpolarizing vasodilators in whole tissue? 3) What kind of K+ and Ca++ channels can be identified in single coronary artery cells? 4) Do the characteristics of these channels support a role for electromechanical coupling in the whole tissue? 5) Are the characteristics of these channels modified by AEP? by hyperpolarizing vasodilators? By providing new insights into the fundamental basis by which contraction is initiated and inhibited in the coronary artery we may aid in understanding the causes of coronary arterial spasm; an increasingly recognized cause of ischemia, heart syndrome and sudden death.